I. Field Of The Invention
This invention relates to a power supply for use with a hybrid laser and, more particularly, to such a power supply for energizing a hybrid laser employed as the transmitting portion of a LIDAR (Laser Infra-red Detection And Ranging) system.
II. Description Of The Prior Art
As known in the prior art, hybrid lasers are employed to provide high energy pulses of coherent wave energy in a single longitudinal mode (and preferably a single transverse mode). Reference is made to U.S. Pat. No. 4,554,666, which issued Nov. 19, 1985. This patent discusses the state of the hybrid laser art in some detail.
Briefly, a hybrid laser is comprised of an optical resonant cavity in which is situated a narrow-bandwidth, low-energy active lasing medium (such as a continuously-excited low pressure CO.sub.2 laser tube) and a wide-bandwidth, high-energy active lasing medium (such as a pulse-excited high-pressure TEA (i.e., transverse-excited atmospheric) CO.sub.2 laser tube). The two laser tubes are arranged in serial relationship with one another within the optical resonant cavity, so that coherent light reflected from a first reflector at one end of the optical resonant cavity must travel through both laser tubes before reaching a second reflector at the other end of the optical resonant cavity. Consequently, the bandwidth of the high-energy coherent wave energy pulses (generated by the high-pressure laser tube) is constrained by the narrow-bandwidth of the low-energy, low-pressure laser tube.
LIDAR, which is similar to radar, employs coherent wave energy at infra-red wavelengths generated by a laser, rather than microwave coherent wave energy at radio wavelengths, to measure the distance to objects, the velocity of such objects by Doppler techniques, etc. As is known, LIDAR is often used for the purpose of making atmospheric measurments of various types.
A CO.sub.2 hybrid laser is particularly suitable for use as the transmitter portion of a LIDAR. This is true because of the ability of a CO.sub.2 hybrid laser to generate single longitudinal mode, high peak power short pulses of infra-red wave energy. The high peak power extends the ranging distance of a LIDAR. The small length of each pulse increases the resolution in range with which objects at slightly different distances may be distinguished, and the single longitudinal mode of the pulse coherent wave energy makes it possible to measure relatively small object velocities with high precision by Doppler techniques. By way of example, it would be desirable to employ a LIDAR comprised of a hybrid CO.sub.2 laser transmitter in a polar-orbit weather satellite for the purpose of making accurate measurements of wind velocity at closely spaced points (every few kilometers) over the face of the earth. Such measured wind velocity data could then be used, along with other weather data, to provide (by computer analysis of the measured data) much more accurate long-term, world-wide weather forecasting than is currently achievable.
An essential requirement of both radar and LIDAR is that no power be radiated by the transmitter during the entire time interval following the transmission of each exploratory pulse during which reflected echoes from objects of interest may be received by the radar or LIDAR. Otherwise, the relatively low-power echo wave energy would be swamped by such undesired transmitter power leaking into the receiver system.
In the past, the low pressure laser tube of the CO.sub.2 hybrid laser transmitting portion of a LIDAR was continuously excited. Therefore, the hybrid laser would normally emit a certain amount of low-power infra-red light, deriving from the low pressure tube laser during the period between successive high-power pulses of infra-red light deriving from the high pressure laser tube. However, in practice, the creation of each high power laser pulse (generated in response to an applied excitation pulse to the high pressure laser tube) results immediately thereafter in the Q of the cavity being spoiled due to optical and thermal effects. This spoiling of the Q of the cavity squelches lasing of both the low and high pressure laser tubes, although the low pressure laser tube continues to be energized and excited (i.e., the plasma current continues to flow therein). It is fortunate that the lasing of the low pressure tube of the hybrid laser of the transmitting portion of a LIDAR takes place, since the production of laser light from the low pressure tube would inevitably be detected in the receiver optics and possibly swamp any received signal.
The problem in the prior art is that after a time interval of roughly 100 microseconds (.mu.s)--highly dependent on laser design--the low pressure tube of the hybrid laser begins to sporadically emit low-power spikes of infra-red light for a certain time before the spoiled Q of the cavity recovers and the low pressure tube resumes continuous lasing. While the total time of the spoiled Q of the cavity to recover is sufficient for object echoes of interest to be received by the receiving portion of the LIDAR, the time interval of only 100 .mu.s is substantially shorter than that which is required to receive the more distant object echoes of interest. Therefore, the problem with the prior art is that the receiving portion of the LIDAR is disrupted by the detection of the sporadic infra-red spikes. The present invention is directed to a solution to this problem.